1. Field of the Invention
The present invention relates to an optical scanning device that scans a scanning surface with a light beam, an optical scanning device control method, and an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, there is known a technology for forming an image on a printing medium such that an electrostatic latent image based on image data is formed by exposing a photosensitive drum on which static charges are formed, the electrostatic latent image is developed by a developer to form a toner image, and the toner image is heated by a fixing roller so as to be fixed to the printing medium.
In such an image forming apparatus, there is known a technology for exposing the photosensitive drum on which the static charges are formed, by scanning the photosensitive drum in the main-scanning direction with a light beam while deflecting the light beam by a polygon mirror rotating at a constant speed.
Meanwhile, a single image forming apparatus may be provided with multiple process line speed modes. For example, different process line speeds may be set in image forming apparatuses of the same model when the image forming apparatuses are shipped.
For another example, it may be desired to set the process line speed in accordance with a parameter of image formation in a single image forming apparatus. As an example in this case, a single image forming apparatus may form images on printing media with different thicknesses. In this case, the image forming apparatus needs to apply more heat for fixing a toner image to a thicker printing medium. The heat to be applied is controlled by setting the process line speed, that is, a printing-medium conveying speed, in accordance with the thickness of the printing medium.
If the process line speed is changed, it is necessary to also change the scanning speed of the light beam in order to maintain the constant exposure amount per unit length in the main-scanning direction on the photosensitive drum. If the image forming apparatus uses a polygon mirror to perform scanning with the light beam, the scanning speed of the light beam can be changed by changing the rotation speed (the rotation frequency per unit time) of the polygon mirror.
If the scanning speed of the light beam Ls changed in the image forming apparatus, scanning time with the light beam per unit length changes and thus the exposure amount per unit length on the photosensitive drum changes. Therefore, to allow the image forming apparatus to achieve the same exposure of the photosensitive drum, it is necessary to change the exposure amount by controlling the light intensity of the light beam according to the amount of change in the scanning speed.
Incidentally, in recent years, to realize a high-speed and high-resolution apparatus, a laser diode array (LDA) that can simultaneously emit multiple (two to four) beams and a surface-emitting laser referred to as a vertical cavity surface emitting laser (VCSEL) that can simultaneously emit about 40 light beams have been developed. Besides, an image forming apparatus, which employs the LDA or the VCSEL as a light source of light beams to expose a photosensitive element and which can form images with higher definition and higher speed, has been put to practical use.
In the exposing optical system using multiple beams an described above, there is known a technology, in which when the process line speed is changed, the number of beams to be used is changed according to the process line speed in order to change the exposure amount without changing the light emission power of the laser.
Japanese Patent Application Laid-open No. 2007-293202 discloses a technology, in which when the process line speed is changed, the rotation frequency of a rotary deflector and the number of light sources are controlled based on a ratio of the process line speed before the change to the process line speed after the change. According to Japanese Patent Application Laid-open No. 2007-293202, it is possible to maintain the rotation frequency of the rotary deflector in the optimal range of the rotation frequency when the process line speed is changed.
Some laser light sources have an upper limit and a lower limit on the available light emission power. In case, there is a problem in that, in the conventional technology, when the rotation speed of the polygon mirror is changed and the light intensity of the light beam is controlled when the process line speed is changed, it may be impossible to achieve the change of the light emission power corresponding to the change in the process line speed. This becomes a big problem particularly when the amount of change in the process line speed is large.
Furthermore, in the exposing optical system using multiple beams, if corrective action is taken by changing the number of beams to be used according to the change in the process line speed, available process line speeds have to depend on the change ratio of the number of beams. Therefore, there is a problem in that it becomes difficult to select the optimal process line speed.
Moreover, even Japanese Patent Application Laid-open No. 2007-293202 does not disclose control such that exposure is performed with the light intensity of the laser light source within the available range and at the optimal process line speed.
Therefore, there is a need to make it possible to appropriately expose the photosensitive drum according to the process line speed.